1 Field of the Invention
The present invention relates to an integration type photodetection system for integrating optical power of light incident to a photosensor to output the integration result.
2 Related Background Art
The conventionally known, integrating luminous energy detecting devices include the integrating photodetectors disclosed in the official gazettes of Japanese Patent Application Laid-Open No. 5-215602, Japanese Patent Application Laid-Open No. 6-18325, Japanese Patent Application Laid-Open No. 6-341899, and WO97/02609. The Integrating photodetectors disclosed in these official gazettes can measure the quantity of light incident to the photosensor by accumulating charge from the photosensor for a predetermined time.
In the integrating photodetectors disclosed in Japanese Patent Applications Laid-Open No. 5-215602, No. 6-18325, and No. 6-341899, however, an integrating circuit for integrating the output of the photosensor is constructed of an active circuit comprised of a power supply, a transistor, etc., and the power must be on during the integration of optical power. This posed the problem that reduction of power consumption was not possible.
On the other hand, WO97/02609 discloses an electron tube which consists of a photocathode and a MOS transistor having a floating gate opposed thereto and which acts as an integrating photodetector. In this electron tube, the gate electrode of the MOS transistor placed in the electron tube is exposed in vacuum so as to face the photocathode. The gate of the MOS transistor is preliminarily charged with predetermined positive charge by tunneling from the substrate, so that a voltage appears depending upon the capacitance between the gate and the substrate (the source and the drain). Since this voltage acts as the gate voltage of the MOS transistor, it can be read out in the form of drain current by placing a predetermined voltage between the source and the drain. When electrons are emitted from the photocathode in accordance with the incident power of light, the electrons fly to the gate maintained at a higher potential than the photocathode, so as to discharge the positive charge stored in the gate. This changes the gate voltage and the gate voltage thus changed can be detected in the form of drain current. At this time, change of the drain current is dependent on the number of electrons emitted from the photocathode, i.e., dependent upon the integral of incident optical power. This integrating photodetector disclosed in WO97/02609 can integrate the incident optical power to some extent even with interruption of the power supplied to the source and the drain, because the gate electrode of the MOS transistor is placed in an insulated state from the substrate.
The integrating photodetector disclosed in WO97/02609, however, had the following problems. Namely, the photodetector can be applicable to only the integration of low incident optical power, because the small capacitance between the gate and the substrate of the MOS transistor is used as a capacitance for the integration of incident optical power. This raised the problems that all the charge was discharged instantaneously with incidence of light of high power and that the detector permitted only the integration of small quantity of light even if the light was received for a long period.
The present invention has been accomplished under such circumstances and an object of the present invention is to provide an integrating photodetection system in which an integral capacity can be set readily according to incident optical power.
In order to solve the above problems, the present invention is directed to an integrating photodetection system for integrating light incident to a photosensor to detect a quantity of the incident light, the photodetection system comprising a main body unit which has a first terminal, and a head unit which has a second terminal attachable to or detachable from the first terminal and which is attachable to or detachable from the main body unit, the head unit comprising the photosensor which is connected to the second terminal and which generates photocurrent according to the quantity of the incident light, and a capacitor which is connected to the photosensor, the main body unit comprising a charging circuit which is connected to the first terminal and which can charge the capacitor while the first terminal is coupled to the second terminal, and a reading circuit which can read a voltage of the capacitor out while the first terminal is coupled to the second terminal, wherein the capacitor is discharged as the photocurrent is generated.
In the integrating photodetection system according to the present invention, the capacitor of the head unit is charged by the charging circuit of the main body unit while the first terminal of the main body unit is coupled to the second terminal of the head unit. After completion of the charging, the quantity of the incident light is measured by the head unit. When light is incident to the photosensor of the head unit, the photosensor generates the photocurrent according to incident optical power. Then the charge accumulated in the capacitor is discharged as the photocurrent is generated. Namely, the incident optical power is integrated as the capacitor is discharged. Once the capacitor is discharged up, it thus becomes impossible to integrate the incident optical power further. However, if a capacitor with a large capacitance is used instead the head unit becomes fit for incidence of light of high power and for reception of light for a long period. In other words, the present invention permits the integrate capacitance to be set according to the incident optical power, by changing the capacitance of the capacitor. After the end of incidence of light to the photosensor, the voltage of the capacitor after the discharge is read out by the reading circuit of the main body unit while the first terminal of the main body unit is coupled to the second terminal of the head unit. Then the quantity of the light incident to the photosensor is computed based on the voltage thus read out.
Preferably, in the present invention, the photosensor is an electron tube comprising a photocathode for emitting photoelectrons according to incidence of the light, and a target of the photoelectrons.
Here, when light is incident to the photocathode of the electron tube as the photosensor, photoelectrons are emitted from the photocathode. Specifically, for example, the target of photoelectrons is connected to the positively charged plate of the capacitor, the photocathode is connected to the negatively charged plate of the capacitor, and the target is kept at a higher potential than the photocathode; whereby the photoelectrons emitted from the photocathode are attracted to the target to induce flow of photocurrent. Then the charge stored in the capacitor is discharged as the photocurrent is generated.
Preferably, in the present invention, the photosensor is a semiconductor device comprised of a pn junction.
Here, when light is incident to the semiconductor device, for example, like a photodiode as the photosensor, photocurrent is generated in the semiconductor device. Then the n-type semiconductor of the semiconductor device is connected to the positively charged plate of the capacitor, the p-type semiconductor of the semiconductor device is connected to the negatively charged plate of the capacitor, and the n-type semiconductor of the semiconductor device is kept at a higher potential than the p-type semiconductor of the semiconductor device; whereby the photoelectrons generated in the semiconductor device migrate toward the n-type semiconductor of the semiconductor device. On this occasion the charge stored in the capacitor is discharged.
Preferably, in the present invention, the head unit comprises a plurality of photosensors and capacitors connected to the photosensors and wherein each of the plurality of photosensors and the plurality of capacitors is connected to the second terminal.
Here, since the head unit comprises a plurality of photosensors and capacitors corresponding thereto, it can measure not only the quantity of the light incident to the head unit, but also illuminance distribution of the light incident to the head unit by contrast of charge amounts stored in the respective capacitors.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.